This invention relates to the production of high purity fused silica glass, and in particular, to methods and apparatus for minimizing the contamination of the silica glass from the dissociation of crown refractory.
In overview, high purity fused silica glass is made by depositing fine particles of silica in a refractory furnace at temperatures exceeding 1650xc2x0 C. The silica particles are generated in a flame when a silicon containing raw material along with natural gas is passed through a fused silica producing burner into the furnace chamber. These particles are deposited and consolidated onto a rotating body. The rotating body is in the form of a refractory cup or containment vessel, which is used to provide insulation to the glass as it builds up, and the furnace cavity formed by the cup interior and the crown of the furnace is kept at high temperatures. In the art, glass-making procedures of this type are known as vapor phase hydrolysis-oxidation processes, or simply as flame hydrolysis processes. The body formed by the deposited particles is often referred to as a xe2x80x9cboulexe2x80x9d and it is understood that this terminology includes any silica-containing body formed by a flame hydrolysis process.
A typical prior art furnace for producing fused silica glass includes an outer ring wall, which supports a refractory crown. The crown is provided with a plurality of burner holes, and each such burner hole is provided with a burner positioned there above at an inlet end for directing a flame through the burner hole into the cavity of the furnace. The furnace is provided with a rotatable base, which with the containment wall forms a cup or containment vessel. The rotatable base, forming the bottom of the cup-like containment vessel, is covered with high purity bait which collects the initial silica particles forming the boule.
The refractory crown, having the burners positioned thereon, functions to trap heat within the furnace. However, since the flame and soot from the burners pass through the burner holes in the refractory crown, the burner holes are maintained at elevated temperatures. Such elevated temperatures in the vicinity of each burner hole cause impurities to leach out of the refractory and produce undesirable dissociation of the refractory which contaminates the silica glass.
If the refractory of the burner hole could be operated at a reduced temperature, it would be possible to reduce the amount of contamination of the glass from the refractory. Also, such reduction in temperature would result in an increase in the internal transmission in the ultraviolet range ( less than 400 nm), thus reducing glass defects and/or contamination. It thus became an object of present invention to reduce the temperature of the refractory material surrounding burner holes in a flame hydrolysis furnace by creating a cooling boundary layer adjacent sidewalls of the burner holes.
The use of gaseous curtains are know for unrelated purposes, such as for preventing the contamination of wafers in semiconductor manufacturing as shown in U.S. Pat. Nos. 4,803,948 and 4,950,156; and for cooling burner nozzles with oxygen as shown in U.S. Pat. Nos. 4,303,386 and 5,567,141. Also U.S. Pat. No. 5,735,928 suggests the use of many layers of gases to provide different types of shields.
It is apparent that there is a need for not only recognizing a problem of contamination of silica glass from over-heated burner hole refractory in high purity fused silica processes, but also for a solution to such problem while maintaining high quality fused silica glass boules.
In view of the forgoing, it is an object of this invention to provide improved method and apparatus for producing silica-containing boules by flame hydrolysis. In particular, it is an object of the invention to reduce burner hole refractory temperatures and virtually prevent the contamination of the silica glass from the refractory. It is a further object of the invention to increase the transmission in the ultraviolet range within the glass and to control glass stoichiometry. These and other objects are obtained by utilizing a porous liner in the burner hole and purging the porous material with a suitable gas to not only cool the liner, but also to create a gas boundary layer that prevents particle buildup in the burner hole and maintains the burner hole at a reduced temperature.